S-190 Unit 6: Critical Fire Weather Summary: Some of the risk involved in fire suppression can be reduced if firefighters understand critical weather events, such as thunderstorms, cold fronts, foehn winds, and the impact they have on fire behavior. Incident Position Description (IPD) Alignment: This unit aligns with the following FFT2 IPD specific duties (https://www.nwcg.gov/positions/fft2/position-ipd): • Apply the knowledge of fuels, terrain, weather, and fire behavior to decisions and actions. Objectives: Students will be able to: • Describe critical fire weather conditions. • Describe critical fire weather events such as cold fronts, thunderstorms, foehn winds, and other local phenomenon that can impact fire behavior. Unit at a Glance: Topic Method Duration Unit Introduction Presentation 5 Minutes Critical Fire Weather Conditions Presentation 15 Minutes Critical Fire Weather Events Presentation 40 Minutes Total Unit Duration 60 Minutes Materials: • Incident Response Pocket Guide (IRPG), PMS 461, https://www.nwcg.gov/publications/461. • NWCG Glossary of Wildland Fire, PMS 205, https://www.nwcg.gov/glossary/a-z. • Notebooks for participants. • Ability to display images and video on large screen. • White board or easel access for group breakout. NWCG S-190 1 of 32 S-190 Unit 6: Critical Fire Weather Slide 1 S-190 Unit 6: Critical Fire Weather S-190 Unit 6: Critical Fire Weather 1 NWCG S-190 2 of 32 Unit 6: Critical Fire Weather Slide 2 Review unit objectives. NWCG S-190 3 of 32 Unit 6: Critical Fire Weather Slide 3 Critical Fire Weather S-190 Unit 6: Critical Fire Weather 3 Play Video Title Critical Fire Weather Summary Description of how geographic area, time of year, and weather contribute to critical fire weather patterns. Time (01:08) Audio NWCG S-190 4 of 32 Unit 6: Critical Fire Weather Slide 4 Critical Fire Weather These weather factors, combined with receptive fuels, may result in extreme fire behavior: • Dry Lightning • Strong & Shifting Winds • Warm Temperatures • Low Relative Humidity • Unstable Atmosphere S-190 Unit 6: Critical Fire Weather 4 Reference Lightning Activity Level (LAL) in the Incident Response Pocket Guide (IRPG), PMS 461, https://nwcg.gov/publications/461. • If fuels are receptive, an interaction with critical fire weather factors can result in extreme fire behavior. NWCG S-190 5 of 32 Unit 6: Critical Fire Weather Slide 5 Critical Fire Weather Changing of Fire Weather Factors Changing Fire Weather Cool Temperatures Hot Temperatures High Humidity Low Humidity Stable Atmosphere Unstable Atmosphere Calm Wind Changing Fire Behavior Strong and Shifting Winds Minimal Moderate Extreme Changing Margin as Fire Weather Factors Change • Greater Margin for Error • Margin for Error • Less Margin for Error • More Options Available • More or Fewer • Fewer Options Available • More Time to Make Options Available • Rapid Decision-Making Decisions Depending on Trends • Significant Risks • Minimal Risks S-190 Unit 6: Critical Fire Weather 5 • It’s important to recognize that changing fire weather conditions can result in changes in the environment and fire behavior. • As changes occur in the fire environment, so does the margin of error for adapting and reacting to those changes. • Firefighters should be aware of changing weather conditions from morning to afternoon, some of which may be extreme: o For example, cool temperatures, high humidity, a stable atmosphere, and calm winds may be present in the morning, but hot temperatures, low humidity, an unstable atmosphere, and strong winds may come ahead of a cold front in the afternoon. o In this example, the margin for error will decrease during the day as fewer options and less time cause rapid decision-making and a significant increase in risks. NWCG S-190 6 of 32 Unit 6: Critical Fire Weather Slide 6 Examples of Weather Phenomena Foehn Winds Cold Front Thunderstorms S-190 Unit 6: Critical Fire Weather 6 Discuss characteristics of these phenomena relative to the local area where the course is being presented. Note to Instructor This is just an introduction to the names of these critical fire weather phenomena. Each will be discussed in more detail in this unit. NWCG S-190 7 of 32 Unit 6: Critical Fire Weather Slide 7 Front There are different types of fronts. This course will focus on the cold front and its impacts on the fire environment. Cold Front • The cold front, symbolized by a blue line with triangles, represents the leading edge of a relatively cold or cooler air mass. S-190 Unit 6: Critical Fire Weather 7 • An approaching cold front can quickly change the fire environment by producing: o Strong and shifting winds. o Warm and sometimes dry air mass. o Unstable conditions. • Historically, firefighter fatalities have occurred during a pre-frontal environment where winds are strong and shifting and the atmosphere is unstable. Reference the Common Denominators of Fire Behavior on Tragedy Fires in the Incident Response Pocket Guide (IRPG), PMS 461, https://www.nwcg.gov/publications/461. NWCG S-190 8 of 32 Unit 6: Critical Fire Weather Slide 8 Pre-Frontal Environment Air mass conditions in the pre-frontal environment can differ based on geographic area. Western U.S. Eastern U.S. S-190 Unit 6: Critical Fire Weather 8 In the western U.S.: • Usually very warm and dry air ahead of the front. This air typically comes from the desert southwest, creating a dry environment. • Unstable air mass ahead of the front. • Overall, the air mass is supportive of an increase in fire behavior. In the eastern U.S.: • Warm and moist conditions typically develop ahead of the front. This air typically comes from the Gulf of Mexico, creating a moist environment. • Overall, the air mass is not supportive of an increase in fire behavior. NWCG S-190 9 of 32 Unit 6: Critical Fire Weather Slide 9 Post-Frontal Environment Air mass conditions may be more supportive of fire behavior and growth in the eastern U.S. compared to post- frontal air masses in the west. Western U.S. Eastern U.S. S-190 Unit 6: Critical Fire Weather 9 Common post-frontal air mass conditions: • Air mass behind the front tends to be more stable than the air mass ahead of the cold front. • Temperatures are cooler. • Relative humidity is higher. NWCG S-190 10 of 32 Unit 6: Critical Fire Weather Slide 10 The Cold Front May promote strong and shifting winds, especially along the frontal boundary. In addition to changes in temperature, relative humidity, and atmospheric stability, the approaching cold front typically results in: Strong & Shifting Winds Wind Speed and Direction Light Moderate Strong S-190 Unit 6: Critical Fire Weather 10 • Winds associated with the passage of a cold front are dangerous, due to the strength of the wind and shift in wind direction as the front approaches and passes through the area. • These strong and shifting winds occur along the frontal boundary and are a result of a strong pressure/temperature gradient between the different air masses. NWCG S-190 11 of 32 Unit 6: Critical Fire Weather Slide 11 Cold Front S-190 Unit 6: Critical Fire Weather 11 Pre-Video Discussion What to expect from a cold front passage: • 150 miles ahead of the front o Wind is usually light and from the southeast direction. o Fire behavior may be active in this environment but predictable. • 50-100 miles ahead of the front o As the front approaches, wind speed increases, and shifts from the southwest. o Wind speed may range from 15 to 30 mph, but could be stronger. o Fire behavior could significantly increase. • Frontal Passage o Wind typically shifts northwest but remains strong (15 to 30 mph) until the front pushes further east. o Expect a gradual decrease in fire behavior as temperature decrease and relative humidity increases. Play Video Title Cold Front Passage and Fire Spread Summary Animation of a cold front moving toward a fire and shifting the wind from northeast to southwest. Time (00:08) No Audio NWCG S-190 12 of 32 Unit 6: Critical Fire Weather Slide 12 Cold Front Environment Characteristics S-190 Unit 6: Critical Fire Weather 12 • Increasing winds along the leading edge and just behind the front. • Pre-frontal conditions are hot, dry, and, unstable – favorable for extreme fire behavior. • Extreme fire behavior may continue for a short period as the front moves through or when temperature decreases and relative humidity increases. NWCG S-190 13 of 32 Unit 6: Critical Fire Weather Slide 13 Knowledge Check Cold Front Scenario It’s March and you’ve been deployed to the State-Line Fire along the border of Colorado and New Mexico. Your crew has been briefed that a cold front is predicted to arrive in the fire area at around 1400. S-190 Unit 6: Critical Fire Weather 13 Play Video Title Cold Front Scenario Summary Animation of a weather map showing a cold front moving over the western states. Time (00:03) No Audio Note to Instructor Students will answer questions on the next two slides based on when the front is predicted to arrive in the area. NWCG S-190 14 of 32 Unit 6: Critical Fire Weather Slide 14 Knowledge Check Cold Front Scenario 1. What are the typical trends in temperature and relative humidity between 1400 and 1700? Increase in temperatures and a decrease in relative humidity. 2. How does the wind speed change as a front approaches? Wind increases as the front approaches. S-190 Unit 6: Critical Fire Weather 14 Question 1: What are the typical trends in temperature and relative humidity between 1400 and 1700? Answer: Increase in temperatures and a decrease in relative humidity. Question 2: How does wind-speed change as a front approaches? Answer: Wind increases as the front approaches.